Brew the espresso coffee and leave it to cool. Use either 15g espresso powder per 60ml coffee or, if using instant coffee dissolve 8 teaspoons of granules/ powder in 240ml water that is just off the boil.
Separate the egg yolks and whites. Put all of the egg yolks into the bowl of a stand mixer with the caster sugar.
Whip the egg yolks and sugar for around 5 minutes until very thick, moussy and pale. (If you do not have a stand mixer then use a medium-sized bowl and electric beaters - it may take slightly longer.) They are whipped sufficiently when a trail remains for a few seconds after the mixture falls off the whisk (this is the ribbon stage).
Beat or stir the mascarpone briefly, just until it is smooth.
Beat the egg white to firm peaks (using clean beaters).
Add the egg yolk mixture to the mascarpone and gently beat it in using the lowest speed setting on your handheld electric beaters. Mix until just smooth being careful not to overmix as the mascarpone could split.
Use a large metal spoon to gently fold the whipped egg white into the mascarpone mixture until no streaks or lumps of egg white remain.
Spoon a small amount of this creamy mixture into the bottom of the serving dish (about 2 tablespoons).
Dip a ladyfinger into the coffee mixture for approximately 3 seconds and lay it on top of the mascarpone layer in one of the glasses. Repeat until the base layer is covered. If necessary, chop the biscuits to make them fit (before soaking them in the coffee).
Crush 2 of the Biscoff cookies and sprinkle them over the ladyfingers.
Warm the Biscoff spread briefly until it is soft enough to drizzle (either in a pan or in the microwave on the defrost setting for around 20 seconds).
Drizzle half of the Biscoff spread over the crushed biscuits.
Now add half of the mascarpone mixture and gently spread it out to cover everything.
Add another layer of ladyfingers dipped in coffee, 2 more crushed biscuits and the remaining Biscoff spread (reheat if necessary).
Top with the remaining mascarpone filling mixture and spread it out.
Cover the dish with food wrap and place it in the fridge to chill and set – for at least 6 hours but preferably overnight.
When you are ready to serve your speculoos tiramisu sieve the cocoa powder over the top and scatter 2 more crushed Biscoff cookies over it.